Guidance for Calculation of Efficiency (PUE) in Data Centers

Overview

PUE is total data center input power divided by IT load power, with 1.0 as the ideal. According to the paper the problem is not the formula but which device lands in the numerator or denominator. The author classifies subsystems across power, cooling and other groups (lighting, NOC, personnel space) in tables, so two facilities are measured within the same boundary.

On the power side, UPS, PDUs/transformers, switchgear, ATS and generator ancillaries (block and jacket-water heaters, battery chargers, controls) count as infrastructure and are included. In cooling, chillers, cooling towers (fans, basin heaters), pumps, CRAC/CRAH units, humidifiers and unit heaters in M&E rooms are infrastructure. Lighting serving the data center is included; NOC and personnel spaces are a judgment call, with the NOC included only when dedicated to the data center.

Practical difficulties get methods. Switchgear and ATS losses, about 1% of total energy, are commonly omitted and should be estimated instead. Shared subsystems, such as a chiller plant that also serves offices, must not be excluded but apportioned by a standardized estimation method. The conclusion is that PUE can be computed without expensive instrumentation; estimation is acceptable if losses are estimated systematically, omission is not. The paper dates from 2015 and should be read alongside ISO/IEC 30134-2 and EN 50600-4-2.

Key points

1. PUE = total data center input power / IT load power; ideal 1.0.
2. UPS, PDUs/transformers, switchgear, ATS and generator ancillaries (block/jacket-water heaters, battery chargers, controls) are infrastructure and included.
3. Chillers, towers (fans, basin heaters), pumps, CRAC/CRAH, humidifiers and M&E-room unit heaters are infrastructure.
4. Lighting serving the data center is included; the NOC is included only when dedicated to the data center.
5. Switchgear/ATS losses (about 1% of total energy) are commonly omitted and should be estimated.
6. Shared subsystems (a chiller plant also serving offices) are apportioned by a standard method, not excluded.

Using it in design

• Build the PUE calculator on an explicit load-classification table: each catalog component is IT, infrastructure, excluded or shared.
• Add switchgear/transformer losses at about 1% of facility energy when not modeled, and include generator block heaters and M&E-room heaters.
• For mixed-use sites ask for a shared_fraction on chiller plants and towers.
• Show which subsystems are inside PUE, provide a NOC toggle, and warn when an infrastructure component has no loss model.

Who should read it

Energy managers, operations teams and designers who report PUE or must compare PUE across two facilities.

Related standards

• ISO/IEC 30134-2
• EN 50600-4-2
• The Green Grid WP#49

Publication details

Authors

Victor Avelar

Publisher

Schneider Electric

Year

2015

Document number

WP 158

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation